In a conventional emergency stop system for an elevator, an output from a tachogenerator for a main rope and an output from a tachogenerator for a drive sheave are compared with each other. If a difference is generated between the outputs, it is judged that a rope slip has occurred. Then, a command for gripping a governor rope is input to a governor rope stop device. When the governor rope is gripped by the governor rope stop device, a safety gear is operated to suddenly stop a car (for example, see Patent Document 1)    Patent Document 1: JP 2004-149231 A